Yeah it depends on what the system is.

Lots of systems can tolerate a lot more downtime than the armchair VPs want them to have.

If people don't access to Instagram for 6 hours, the world won't end. Gmail or AWS S3 is a different story. Therefore Instagram should give their engineers a break and permit a migration with downtime. It makes the job a lot easier, requires fewer engineers and cost, and is much less likely to have bugs.